SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Components of Other Income, Net (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Summary of Accounting Policies [Line Items]
Other income, net	$ 71,561	$ 34,201
Pension, SERP and PBOP Non-Service   Income Components
Summary of Accounting Policies [Line Items]
Other income, net	54,300	20,100
AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	9,900	9,200
Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	400	3,700
Investment (Loss)/Income
Summary of Accounting Policies [Line Items]
Other income, net	(200)	(600)
Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	6,700	1,500
Other
Summary of Accounting Policies [Line Items]
Other income, net	500	300
CL&P
Summary of Accounting Policies [Line Items]
Other income, net	19,564	4,908
CL&P | Pension, SERP and PBOP Non-Service   Income Components
Summary of Accounting Policies [Line Items]
Other income, net	15,900	2,700
CL&P | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	2,800	1,700
CL&P | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	0	0
CL&P | Investment (Loss)/Income
Summary of Accounting Policies [Line Items]
Other income, net	(400)	400
CL&P | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	1,300	100
CL&P | Other
Summary of Accounting Policies [Line Items]
Other income, net	0	0
NSTAR Electric
Summary of Accounting Policies [Line Items]
Other income, net	29,231	16,812
NSTAR Electric | Pension, SERP and PBOP Non-Service   Income Components
Summary of Accounting Policies [Line Items]
Other income, net	21,000	10,100
NSTAR Electric | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	4,900	6,200
NSTAR Electric | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	0	100
NSTAR Electric | Investment (Loss)/Income
Summary of Accounting Policies [Line Items]
Other income, net	(300)	200
NSTAR Electric | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	3,500	100
NSTAR Electric | Other
Summary of Accounting Policies [Line Items]
Other income, net	100	100
PSNH
Summary of Accounting Policies [Line Items]
Other income, net	7,509	4,166
PSNH | Pension, SERP and PBOP Non-Service   Income Components
Summary of Accounting Policies [Line Items]
Other income, net	6,600	2,700
PSNH | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	400	600
PSNH | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	0	0
PSNH | Investment (Loss)/Income
Summary of Accounting Policies [Line Items]
Other income, net	200	100
PSNH | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	300	700
PSNH | Other
Summary of Accounting Policies [Line Items]
Other income, net	$ 0	$ 100